go.crypto/ssh: improve error message when no authentication methods remain
go.crypto/ssh: improve error message when no authentication methods remain

R=golang-dev
CC=golang-dev
https://golang.org/cl/5960044

mattharden/crypto,Carrotman42/crypto,juniorz/crypto,CodyGuo/crypto,cheffo/crypto,cheffo/crypto,bford/golang-x-crypto,Carrotman42/crypto,kim-racktop/crypto,conseweb/golangcrypto,ScriptRock/crypto,golang/crypto,cmars/tools,kim-racktop/crypto,wooutl/golangcrypto,golang/crypto,ScriptRock/crypto,pki-io/crypto,pki-io/crypto,lucevers/crypto,tca/crypto,cmars/tools,conseweb/golangcrypto,CodyGuo/crypto,cmars/tools,thomaso-mirodin/crypto,a-palchikov/crypto,juniorz/crypto,guaiguaihw/crypto,golang/crypto,mattharden/crypto,wooutl/golangcrypto,cmars/tools,a-palchikov/crypto,bford/golang-x-crypto,zj8487/crypto,lucevers/crypto,zj8487/crypto,thomaso-mirodin/crypto,tca/crypto,progrium/crypto,progrium/crypto,guaiguaihw/crypto